The invention is based on patent application No. 11-261932 Pat. filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method and an apparatus for forming a thin film.
2. Description of the Background Art
Thin films have been produced for various purposes by various methods, and a plasma CVD method is one of typical methods of forming thin films.
A silicon film is an example of the thin film formed by the plasma CVD method. This silicon film is a material of a TFT (Thin-Film Transistor) switch provided in each pixel, e.g., of a liquid crystal display. A silicon oxide film and a silicon nitride film as well as a silicon-containing thin film used, e.g., in a solar battery are also examples of the thin films.
For example, the silicon film formation by the plasma CVD method is performed in such a manner that a silane gas and a hydrogen gas are mixed together, and a radio-frequency power is applied to an electrode opposed to a deposition target substrate for causing decomposition and dissociation of the gas mixture so that an amorphous silicon film is formed on the substrate.
In the case of the silicon oxide film, an oxygen-containing gas such as an oxygen gas is mixed with a silicon-containing gas such as a silane gas, and the radio-frequency power is likewise applied to cause dissociation and decomposition of the gas mixture so that the silicon oxide film is formed on the substrate.
FIG. 4 is a schematic cross section of an example of a parallel plate type plasma CVD apparatus in the prior art.
A parallel plate type plasma CVD apparatus B shown in FIG. 4 includes a vacuum casing Cxe2x80x2, which contains a deposition chamber 9, a flat plate electrode 91 having a plurality of gas-passage apertures, and a gas retaining chamber 92 connected to the deposition chamber 9 via the plate electrode 91.
The casing Cxe2x80x2 further includes a gas supply pipe 93 for supplying a deposition gas (i.e., a gas for film deposition) into the gas retaining chamber 92, a radical material gas supply pipe 94 for supplying a radical material gas into the gas retaining chamber 92, a gas outlet 95, a substrate inlet/outlet opening 96 provided with a gate value GAxe2x80x2 for transferring the deposition target substrate Sxe2x80x2 into and from the chamber 9, and a substrate holder 97 which is vertically movable and can hold the deposition target substrate Sxe2x80x2 during the deposition. The holder 97 is internally provided with a heater H for heating the substrate. The electrode 91 is opposed to the substrate holder 97, and the gas retaining chamber 92 is located above the electrode 91. An electrically conductive porous plate 98 for preventing excessive generation of gas plasma in the gas retaining chamber 92 is arranged above the electrode 91.
A vacuum pump or gas discharging device 951 for achieving a predetermined low pressure in the deposition chamber is connected to the gas outlet 95.
The substrate holder 97 and the vacuum casing Cxe2x80x2 are grounded. In the plasma CVD apparatus B, the deposition gas and the radical material gas supplied thereto are mixed in the gas retaining chamber 92, and are supplied into the deposition chamber 9 through the conductive porous plate 98 and the parallel plate electrode 91. The electrode 91 is connected to a radio-frequency power source PS9 via a matching box MB9. When turned on, the power source PS9 supplies the power to the electrode 91 so that the plasma PL9 of the gas mixture is formed, and the film can be formed on the deposition target substrate Sxe2x80x2.
However, when the gas mixture formed of the gases which have different dissociation energies, respectively, is excited by the radio-frequency power, the gas molecules (e.g., silane molecules) having a lower dissociation energy are dissociated with a higher priority, and the degree of dissociation of hydrogen- or oxygen-containing gas is low.
When forming the silicon film or the silicon oxide film, a higher density of the hydrogen radicals or oxygen radicals having a low energy can provide the film of a high quality. However, the conventional plasma CVD cannot achieve a high density of the hydrogen or oxygen radicals, and therefore cannot provide the film of a good quality.
For increasing the density of the hydrogen radicals or oxygen radicals, the radio-frequency power for forming the gas plasma may be increased. However, this increases the plasma potential, and causes excessive dissociation of the silane gas so that fast ions are produced, and therefore damages by ion collision are caused in the film. Accordingly, it is impossible to form the film of a low defect and a high quality.
An ECR-CVD method has been proposed as a method for obtaining the good quality. In this method, a microwave dissociates a gas such as a hydrogen or oxygen gas having a high dissociation energy in an ECR plasma source, and hydrogen or oxygen radicals are emitted to the substrate while supplying a silane gas to a portion near the substrate so that the silicon film or the silicon oxide film is formed on the substrate.
According to this method, a film of a good quality can be obtained without causing excessive dissociation of the silane gas. However, it is impossible to form a uniform film over a large area due to the structure of the ECR plasma source, and it is impossible to satisfy a current demand for increase in size of the deposition target substrates.
An object of the invention is to provide a thin film forming method and a thin film forming apparatus, in which a deposition gas and a radical material gas having different dissociation energies are used for forming a film, and dissociation of each gas is controlled to suppress generation of a large amount of ions due to excessive dissociation of each gas as well as a high plasma potential causing damages on the films so that the film of a high quality can be formed uniformly over a large area.
The invention provides the following thin film forming methods and apparatuses.
(1) Thin Film Forming Method
A thin film forming method for forming a predetermined thin film on a deposition target substrate, including the steps of:
preparing a deposition chamber provided with a substrate holder, and a radical emitting device continuing to the deposition chamber for emitting neutral radicals uniformly to a whole deposition target region of the deposition target substrate held by the substrate holder;
arranging the deposition target substrate on the substrate holder;
forming deposition gas plasma at the vicinity of the deposition target substrate arranged on the substrate holder by supplying a predetermined deposition gas into the deposition chamber; and
producing neutral radicals by exciting and dissociating a predetermined radical material gas in the radical emitting device, and uniformly emitting the radicals to the deposition target region of the deposition target substrate.
(2) Thin Film Forming Apparatus
A thin film forming apparatus including:
a deposition chamber;
a radical emitting device continuing to the deposition chamber;
a substrate holder arranged in the deposition chamber; and
a deposition gas plasma producing device for forming plasma of a predetermined deposition gas at the vicinity of a deposition target substrate arranged on the substrate holder, wherein
the radical emitting device is opposed to a whole deposition target region of the deposition target substrate arranged on the substrate holder, produces neutral radicals by exciting and dissociating a predetermined radical material gas, and emitting the radicals uniformly to the whole deposition target region of the substrate.
According to the thin film forming method and apparatus of the invention, the deposition target substrate is arranged on the substrate holder in the deposition chamber. The plasma is formed from the deposition gas supplied into the deposition chamber. The radical emitting device is supplied with the radical material gas, and the neutral radicals are produced therefrom by exciting and dissociating the radical material gas independently of formation of the deposition gas plasma. A predetermined thin film is formed on the deposition target substrate by the deposition gas plasma and the neutral radicals emitted from the neutral radical emitting device.
For forming the thin film, the deposition gas and the radical material gas having different dissociation energies are used. These gases are excited and dissociated independently of each other in the deposition chamber and the radical emitting device, respectively. Accordingly, the dissociation of each gas is controlled to maintain the low plasma potential near the deposition target substrate, and generation of fast ions and fast electrons as well as injection thereof into the deposition target substrate are suppressed. A desired plasma density is maintained near the deposition target substrate, and also the neutral radicals required for the deposition can be efficiently emitted into the deposition target substrate. This enables formation of the film of a high quality.
Furthermore, the neutral radicals are emitted from the radical emitting device uniformly to the whole deposition target region of the deposition target substrate, and thereby a predetermined thin film can be formed uniformly over a large area of the large deposition target substrate.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.